An imaging system generally needs the capability to adjust the focus to compensate for varying object distances. There are number of available techniques for performing this function. These techniques will primarily be described in terms of refractive imaging systems using lenses; however, equivalent reflective elements can often be substituted.
A first, and probably the most common, technique is to simply mechanically move the lens elements along the optical axis. The focus adjustments on a camera, microscope, or telescope are typical examples. There are numerous variations on mechanical focusing. The primary drawback of this technique is bulk. Large variation in lens position is required in order to accommodate large variations in object distance.
A second technique uses segmented optics with individual actuators on the segments. This approach is primarily seen in reflective optical systems such as large telescope mirrors. Segmented optical systems are primarily used in adaptive optics systems to compensate for variable distortions caused by effects such as atmospheric turbulence. Some limited focusing can be achieved with this technique. Clearly, the mechanical and control aspects of a segmented optic is very complex.
A third technique uses a liquid filled, elastic lens in which mechanical pressure is used to deform the curvature of the lens. This technique is exemplified by U.S. Pat. No. 7,142,369. Basically, the technique seeks to mimic the focus mechanism used in the eye. The main difficulties with this approach are that the lens quality is highly dependent on the elastic uniformity of the lens material and that the curvature achieved is not optimal for imaging. As with the eye, image quality is only good for objects near the center of the field of vision and non-uniform imaging, e.g. astigmatism, can occur. For larger lenses, gravity or acceleration can significantly affect the imaging quality by causing the lens to sag.
A fourth technique also uses a liquid lens approach. In this technique, two, non-mixing liquids are used, e.g. oil and water. An interface is formed between the liquids that can be deformed by applying an electric field. A small lens with a roughly one-to-one diameter to thickness ratio can be formed in this fashion. Large thin lenses are impractical with this approach due to fabrication constraints and gravity or acceleration effects.
A fifth technique utilized the ability to change the refractive index of some material by applying an electric field. This approach is exemplified in U.S. Pat. No. 7,215,480. Only small changes in refractive index can be obtained by this technique, which means only small changes in focus can be achieved.